The invention relates to a filter for very short electromagnetic waves, consisting of a plurality of resonators forming filter circuits and which are coupled to one another, are operated in the dual-mode, and whose first and last filter circuits in the direction of the transmitted energy are provided with connection lines for the supply and discharge of the electromagnetic energy. An additional coupling is provided between at least two filter circuits which do not directly follow one another in electrical mode of operation and the individual resonators are arranged in the form of rows next to one another.
Filters in micro-wave technology are, as is known, constructed from a plurality of micro-wave resonators which are coupled to one another, the coupling of which can take place either capacitively or inductively. The resonators themselves can consist, for example, of so-called coaxial line resonators or wave guide resonators.
In contrast to filters constructed with concentrated or lumped circuit elements, as a result of the geometrically predetermined configuration of the resonators it is not possible for every circuit which can be constructed in the concentrated technique to be readily transferred to the micro-wave frequency region. This difficulty occurs, in particular, when it is necessary to produce attenuation poles in the attenuation characteristic of the filter and/or a transit time leveling in the pass band of the filter by means of additional couplings of filter circuits. This difficulty is eliminated by means of the arrangement, described in the German OS 1 942 867, of resonators in adjacent rows with additional over-couplings in the common partition wall of two resonators arranged in different rows.
The possibility is also known of constructing micro-wave filters with cavity resonators which are simultaneously operated in more than one mode ("micro-wave filters employing a single cavity excited in more than one mode", "Journal of Applied Physics", Vol. 22, No. 8, August 1951 by Wei-Guan Lin; "A Four Cavity Elliptic Waveguide Filter", "IEEE Transactions on Microwave Theory and Techniques", Vol. -MTT. 18, No. 12, December 1970 by Williams, A. E.). Here preferably two identical but orthogonal loads are employed in H.sub.101 - or H.sub.111 - resonators and are coupled to one another by means of a coupling screw arranged at 45.degree. to the direction of the E-vectors (dual mode). In this way two electric oscillating circuits of a filter can be constructed in a technically effective fashion in one single cavity resonator. On account of the reduction in weight and volume of up to 50%, an important field of application consists in satellite technology, particularly since high electrical requirements are made on the filters employed therein which become manifest in a relatively large number of electric oscillating circuits.
As these filters also require attenuation poles and/or a leveling of the transit time in the pass band, it is obviously desirable to find suitable filter circuits in the dual mode technique for this purpose. In this connection a proposed construction is known ("Nonminimum-Phase Optimum-Amplitude Band pass Waveguide Filters", "IEEE Transactions on Microwave Theory and Techniques", Vol. MTT-22, No. 4, April 1974 by Atia, A. E. and Williams, A. E.), which, however, is restricted to filter circuits which are symmetrical both with respect to structure and with respect to element values. Furthermore they exhibit additional couplings which frequently overlap, and cannot be preselected with respect to number and geometric position within the filter arrangement. Also, the number of electric oscillating circuits of the filter circuits must amount to a multiple of 4 so that this proposal frequently cannot be practically realized.
A possibility of improving the realizability of filter circuits which are asymmetrical, particularly with respect to element values and which can be operated in the dual mode has been disclosed by the German OS 2 511 800 and consists in arranging the resonators in adjacent rows and providing a different number of resonators in the rows.
The coupling of filter circuits in spatially different resonators is subject to the condition that the relevant two filter circuits should be spatially orientated in like manner, so that e.g. their E vectors run parallel with one another. This condition restricts the number of theoretically conceivable couplings and thus the spectrum of possible realizations or permits realizations only without the use of additional couplings, which in themselves are desirable, or only with production technology disadvantages.